Macroinvertebrate Structure and Drift in the Blanco River: A Karst Texas Stream Subject to Hydrologic Variability

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<p>Patterns in benthic macroinvertebrate communities were assessed
within the Blanco River drainage of central Texas to determine
structuring mechanisms related to abiotic and biotic influences of
the watershed. The Blanco River is a flashy karst system and
naturally fragmented because of water loss to the Trinity and
Balcones Fault Zone Edwards aquifers and because of unique
geological formations in the middle reaches. Seven mainstem sites,
two tributary sites, and two seep sites were sampled qualitatively
and quantitatively within the Blanco River drainage seasonally from
October 2003 through July 2005. In addition, 24-h drift samples
were taken in spring, summer and fall from three sites.
Ephemeroptera, Trichoptera and Diptera were the most abundant taxa
in benthic and drift samples, although riffle beetles (Coleoptera)
and Argia (Odonata) were among the most ubiquitous. Habitat, site,
and season explained 33% of community variation in the drainage (F
= 1.35, P &lt; 0.01). Current velocity and substrate were primary
habitat factors associated with taxa occurrence and distribution.
Adjacent communities differed (analysis of similarity: R = 0.086, P
= 0.02) among mainstem sites suggesting local habitat factors were
more important than spatial factors in determining community
structure. Tributary and seep communities were similar through time
and provided habitat for several taxa not found in the mainstem and
habitat for source populations for many mainstem taxa. Biodiversity
of the Blanco River watershed is dependent upon the diversity and
persistence of mainstem, tributary, and seep habitats. However,
structure and function of the macroinvertebrate communities and
their habitats are susceptible to degradation by increasing surface
and groundwater withdrawals, urban development, and water detention
devices such as low-head dams.</p>